This application claims priority Japanese Patent Application No. 2001-025191, filed Feb. 1, 2001 in Japan, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to an atomic fountain apparatus, especially to a cesium atomic fountain apparatus.
Frequency standards using cesium atoms have been widely used hitherto because of their high precision. With the progress of technologies in recent years, their accuracy requirements have been more and more strict, and more accurate frequency standards have been demanded.
2. Description of the Related Art
FIG. 9 shows the operating principle of a prior art beam-type cesium frequency standard. In FIG. 9, reference numeral 80 refers to a container, 81 refers to a microwave resonator, 82 refers to a cesium atomic-beam, and 83 refers to a microwave, respectively.
When the cesium atomic beam 82 is input into the microwave resonator 81, the cesium atomic beam 82 interacts with the microwave 83, causing the cesium atoms having two energy levels to resonate with the frequency of the microwave. The cesium atoms are allowed to jump from one energy level to the other energy level by the resonance. The frequency of the microwave resonating with the cesium atoms is approximately 9.192xc3x97109 Hz (approximately 9 GHz) which provides a standard of time for an atomic clock. With this standard, an error of one second is caused in several millions of year (1014xcx9c1015 seconds).
Because the atoms whose state was altered with the resonance absorb a light, this state can be detected, for example, by irradiating a light. When no resonated, the atoms do not absorb the light. When irradiated a light to an atom of which energy state is changed by the micro wave resonance, the light is absorbed and fluorescent light is emitted. However, in an atomic of no resonant state, the fluorescent light is not emitted.
In the conventional beam type frequency standards, frequency shifts or frequency fluctuations often occur due to the Doppler effect and other factors. As is well known, there are two kinds of the Doppler effect, one is the primary effect caused by moving, and another is the secondary effect based on the relativity. According to the quantum theory, each of the energy levels of cesium atom, which usually take discrete values, has a uncertainty width, which tends to be reduced with increases of interaction time (measuring time). Having an uncertainty in each energy state has an uncertain width may cause the frequency fluctuation within a certain width of Lorentz distribution, posing an accuracy problem.
Recent research and development efforts for improving such standards are aimed mainly at an atomic fountain type standard. This type of technology has been realized by the progress of laser cooling technology, which may produce gas atoms cooled to very low temperatures of mean velocity of a few centimeter per second equivalent to a few xcexcK. By using such cryogenic atoms, not only a very long interaction time can be obtained, but also frequency shift due to the secondary Doppler effect can be reduced, so that high accuracy frequency standards can be realized. In such a case, because neutral atoms cannot be held at the same position such as by interaction in ion traps, the cesium atoms are tossed up vertically so as to pass through the microwave resonator. This method of tossing up atoms is called the atomic fountain type (or the atomic fountain system).
The atomic fountain type is characterized in that the spectral line width can be very narrow and the Doppler effect can be reduced by using atoms whose velocity ( less than 5 m/sec) is considerably slower than that (250 m/sec) in the beam-type frequency standards.
Slow atoms can be realized by laser cooling. The laser cooling is a cooling method of atoms by using forces that the atoms receive, when absorbing or emitting a light. The cesium atoms can be cooled to temperatures near absolute zero, using the laser cooling. When an atom is irradiated with a laser beam, the atom absorbs the light and receives a force in the direction of the light traveling, and ground state electrons of the atom are excited. The electrons fall to the ground state, emitting fluorescent light uniformly in all direction. Because the momentum is always conservative in each direction, which means the atom receives a force in the reverse direction of the laser irradiating direction. Using the effect, the movement of the atom can be controlled to be still by laser irradiating from each positive and negative directions of x, y and z axis.
When irradiated by two laser beams of a frequency slightly below the resonance frequency in opposite directions, atoms absorb laser beam in one direction and do not absorb laser beam in the other direction under the influence of the Doppler shift. As the result, the atoms receive forces so that the atoms come to a halt, even if they are moving in any direction. Thus the temperature of the atoms is lead to a drop.
FIGS. 10A, B and C shows drawings explaining the atomic fountain type. Now assume that a certain velocity is given to an atom "khgr", and a laser beam of a frequency of xcexdxe2x88x92xcex94xcexd+xcex4N is applied to the atom in one direction, while another laser beam of a frequency of xcexdxe2x88x92xcex94xcexdxe2x88x92xcex4N is applied to it in the other direction, as shown in FIG. 10A. At this time, the velocity of the atom "khgr" becomes zero when viewed from a person who is still on the coordinates moving at a velocity of xcexd0=cxcex4N/N (that is, when viewed from a moving person), where c is the velocity of light. In other words, when viewed from a person in the laboratory, the velocity of xcexd0 is given to the atom "khgr".
In practice, laser cooling is carried out in six directions, and cesium atoms are tossed upward (like a fountain) at a velocity of xcexd0 by changing the frequency in the vertical direction, as shown in FIG. 10B. FIG. 12C shows the atomic fountain of the tossed cesium atoms up, which pass through a microwave generator.
FIG. 11 is an external view of a conventional atomic fountain type cesium frequency standard. In the figure, reference numeral 90 refers to a magnetic shield, 91 refers to a uniform field generator, 92 refers to a microwave resonator, 93 refers to a magneto-optical trap, 94 refers to an input section of a laser beam applied to cesium atoms in six directions in a magneto-optical trap, 95 refers to a signal detector, and 96 refers to an ion pump, respectively. Tossing the cesium atoms in the vertical direction can be realized by a resultant forces of vertical direction components of forces caused by laser beams from four directions of the input sections of the laser beam 94.
The atomic fountain is accomplished by three steps of laser capture (trap), cooling and vertical launch. As the trap of the atoms, a magneto-optical trap 93, which traps cesium atoms by irradiating with laser beams in six directions in an inhomogeneous magnetic field which has a minimum magnetic field, is used. The captured atoms are cooled by polarized gradient cooling to a temperature below the Doppler limit (laser cooling). Polarized gradient cooling is carried out by using an optical molasses comprising six laser beams having the same frequency. Furthermore, when the frequency of the laser beam irradiated in a downward direction is set less than the frequency of the laser beam irradiated in a upward direction, a moving molasses can be realized, that is, the atoms can be tossed upward while maintaining very low temperatures. The atoms pass twice through the microwave resonator 92 disposed on the upper part, once the way up and once the way down, and a Ramsey resonance signal is observed in the signal detector 95 placed under the magneto optical trap 93. In the atomic fountain type, a spectral line width as narrow as approximately 1 Hz can be obtained because the interaction time is a period the atoms float in the microwave resonator 92.
Problems associated with the aforementioned conventional atomic fountain type will be described in the following, referring to an external view of the conventional atomic fountain type cesium frequency standard shown in FIG. 10. The atoms launched under thee microwave resonator 2 pass through a hole of an approximate 1-cm diameter hole provided on the microwave resonator 92 and continue traveling upward to a top at which the energies is lost to fall down. Since the atoms passing through the hole and moving upward shift in the horizontal direction because of the horizontal components of the velocity, not all of the descending atoms return to the position of the hole on the microwave resonator 92, with only about 10% of them actually returning to the hole. The signal detector 95, on the other hand, detects only those atoms which fall down and passing through the hole on the microwave resonator 92 again among the atoms which have been launched and passed through the hole. As the result, the conventional atomic fountain type has an essential problem that the detected spectrum signal is so small that the S/N ratio is not enough.
An object of the present invention is to provide an atomic fountain apparatus that can improve the S/N ratio of the spectrum by suppressing the diffusion of the launched atoms in the horizontal direction.
In the present invention, the launched atoms are irradiated with a laser beam in the direction of the launched atoms to collimate the atoms. Irradiated continuously with the collimating laser beam, the atoms hardly diffuse horizontally, however the presence of the field of light may shift the observed frequency for measuring atoms. It is a problem to be solved.
Another object of the present invention is to solve the problem for the atomic fountain apparatus.
Atomic fountain apparatus of the present invention comprises a collimation laser generating section for generating a laser beam of a frequency that does not resonate with the atoms. The collimation laser beam output by the collimation laser generating section is applied to the atoms in the direction of the tossed atoms.
Moreover an atomic fountain apparatus for laser trapping, cooling and tossing atoms with a plurality of laser beams and comprising a microwave generator. The atoms passe upward and fall back through a microwave resonator are observed. The atomic fountain apparatus comprises a collimation laser generating section for generating a laser beam of a frequency that does not resonate with the atoms. Further it comprises a switch for controlling on and off of the irradiation of the laser beam output from the collimation laser generating section. The collimation laser beam output by the collimation laser generating section is applied in the direction of the tossed atoms. The switch is turned off before the atoms reaches the microwave resonator.
The present invention allows almost all the launched atoms to return to the hole of the microwave resonator by reducing the horizontal velocity component of the atomic fountain using the dipole force generated by the electrical field of the laser beam. The S/N ratio is improved by the collimation of the atoms.
According to the present invention, the velocity component in the direction vertical to laser beam is suppressed using a dipole force caused with a laser beam, so it is possible to improve the S/N ratio and consistently guarantee an accuracy of one second error in several million years.
The objects, advantages and features of the present invention will be more clearly understood by referencing the following detailed disclosure and the accompanying drawings.